1. Field of the Invention
This invention relates to a method of heat treating beads or pellets of thermoplastic substance prior to processing the resulting material. It is more particularly concerned with such a method which does not require electrical heating of the pellets, in which the process is operated in one of three operating modes, depending upon the desired drying requirements and the actual operating conditions.
2. Description of the Prior Art
Electrically heated dehumidifying dryers are generally used in the plastics industry. The reason for using electrical units is their ability to be readily controlled at a setpoint over a wide temperature range. The electrical heating elements can be controlled so as to be operated anywhere between 0 and 100% of load. The heating range for the plastic pellets or beads generally used is about 130.degree. F. to 400.degree. F. and control is readily achieved by simply controlling the current in the heating element. The cost of heating the plastic material by electrical means is considerably greater than the cost of generating an equivalent amount of heat by combustion of natural gas, oil, or other combustible fuel. Prior combustion heating devices, however, cannot be as simply controlled over a wide range. The heating of the pellets is generally accomplished by passing heated air from a heat exchanger through them. The blower required to move the air through the pellets necessarily introduces some heat. A desiccant, such as a molecular sieve, is frequently used to facilitate moisture removal from the air current and its heat of absorption is passed on to the air in the system.